Over the past several years, attempts have been made to design a sanitary napkin having improved adsorbent utilization and controlled staining and flow characteristics. Sanitary napkins are traditionally made from inexpensive cellulosic material. While these materials do provide absorbency when used without modification, they suffer from certain disadvantages. Menstrual fluid is not immobilized in wood pulp fluff or other conventional cellulosic absorbents. Because of this factor, when pressure is exerted which compresses these absorbents, the fluid is liberated and can either be pushed upward through a fluid permeable covering to contact the wearer or can run outward along the top surface of the absorbent and may promote side staining.
In an attempt to counteract these difficulties, fluid directional means have been used. These directional means which have been designed to move fluid rapidly through the top surface of absorbent material and/or directionally outward toward the ends of the napkin have tended to increase the absorptive efficiency and capacity of a napkin but, even with these improvements, saturated napkins still may liberate menstrual fluid.
Attempts have been made to at least partially immobilize menstrual fluid and increase the absorptive capacity of napkins by the inclusion of so-called superabsorbent material. This material is either modified cellulose or synthetic polymer with increased capability for absorbing aqueous solutions. The superabsorbent material is generally in the form of a cross-linked three dimensional structure which allows for the penetration of the aqueous component of menstrual fluid but the suspended plasma proteins and blood cells are too large to penetrate into this preformed superabsorbent material structure and are left as a partially hydrated and concentrated mass surrounding the particular superabsorbent material particle.
The interaction with superabsorbent material is further complicated in that the blood cells and plasma proteins present in menstrual fluid also appear to have a large affinity for the aqueous portion of the blood. This is thought to be the case because it has been heretofore impossible to dehydrate blood to such an extent that precipitation of the solids via the withdrawal of fluid by superabsorbent material occurs. Thus a balance in equilibrium is determined in any particular napkin by the relative magnitude of the affinities of the superabsorbent materials and the blood solids for the aqueous portion of the blood as well as the relative absolute amounts of each component.
While superabsorbent material traditionally absorbs the aqueous portion of blood containing fluids with extreme rapidity, initially the protein fraction of the blood surrounds the individual superabsorbent particles, because of this preferential absorption therefore the absorption of the fluid as a whole is decreased.
There have been a variety of other compounds and approaches utilized in an attempt to immobilize menstrual fluid and/or blood. One such approach is set forth in U.S. Pat. No. 3,669,103 which describes cross-linking superabsorbent materials of different chemical compositions. The result is a covalently cross-linked gel which can absorb aqueous fluid. The structure formed by this reaction is a three-dimensional polymeric chain network which is water-insoluble but, water-swellable. In blood, the swollen gel is surrounded by a viscous, partially dehydrated, blood solid residue.
Another attempt to immobilize blood or menstrual fluid is disclosed in U.S. Pat. No. 3,670,731 which teaches the utilization of cross-linked hydrocolloids such as hydrolyzed, cross-linked polyacrylamide or sulphonated polystyrene. Cross-linking agents are di-vinyl chlorides and these compounds are the typical covalently cross-linked three-dimensional network gels associated with typical superabsorbent systems.
Another example of this type of system is disclosed in U.S. Pat. No. 3,645,836 which discloses the preparation of "gel-forming fibers" by precipitating guar gum from water with isopropyl alcohol. This reaction would hardly form a true gel because it is difficult to see how the water-soluble guar gum molecules could become cross-linked except, perhaps, by mechanical entanglement. After absorbing aqueous fluid the fibrous material would most likely turn into a viscous solution rather than a true gel.
Yet another approach is that disclosed in U.S. Pat. No. 3,810,468 which describes a lightly cross-linked, water-insoluble superabsorbent material. Example 8 describes "various copolymers of mono-olefinic compounds with maleic anhydride which can be lightly cross-linked and reacted with ammonia or alkali to produce water-insoluble, highly water-swellable polymer products". Mono-olefinic compounds are ethylene, styrene or vinyl methyl ether. The patent further teaches that cross-linkages are introduced in the monomer charge or, "alternatively, the olefinicmaleic anhydride copolymer is prepared as a substantially linear polymer and then reacted with a cross-linking agent." In order to render the copolymer hydrophilic the material is placed in a pressure vessel and treated with ammonia gas to convert the maleic acid to the "half amide-half ammonium salt form".
U.S. Pat. Nos. 3,980,563 and 4,154,898 describe superabsorbent materials of differing chemical compositions including a maleic anhydride copolymer. All of these materials are slightly cross-linked, water-insoluble but water-absorbing compounds.
Another similar type of compound system is described in U.S. Pat. No. 3,983,095 which pertains to the preparation of water-insoluble water-swellable derivatives of a copolymer of maleic anhydride and at least one kind of suitable vinyl monomer in fiber form (fiber form exists when in the dry state). The fiber will form the same type of gel with aqueous fluid after it has been absorbed as those described above. Other patents of interest which form the same type of cross-linked three-dimensional gel structure involving water-insoluble materials are U.S. Pat. Nos. 4,192,727 and 4,051,086.
Another approach is that disclosed in U.S. Pat. No. 4,179,416 which relates to the formation of alloy fibers where a synthetic polymer is dispersed in a matrix of regenerated cellulose. The synthetic polymer is water-loving and non-crystalline. Because of these characteristics the fluid retention of the alloy fiber is larger than the pure regenerated cellulose fiber. One of the synthetic linear polymers mentioned is Gantrez AN-149 manufactured by GAF Corporation. Gantrez is a methyl vinyl ether/maleic anhydride copolymer. According to the procedure set forth in this procedure, Gantrez will become completely hydrolyzed after injection in the viscose dope and in the regeneration bath. In addition, the hydrolyzed Gantrez will be completely enveloped by the regenerated cellulose matrix and therefore, in this disclosure it could not react directly with blood or menstrual fluid.
With the exception of the last mentioned patent, all of the prior art approaches have been directed towards forming systems which are completed disordered covalent polymer networks. This system is described and categorized by J. P. Flory in an article in Far. Disc. Chem. Soc. 57; 7-18, 1974. In this article Flory develops a classification scheme built upon the structural criteria of the compounds used for forming various gelling agents. All of the prior art with the exception noted above, is categorized as a class 2 gel by the Flory classification.
Interestingly enough, U.S. Pat. No. 3,810,468 discussed previously discloses a lightly cross-linked polymer including an olefinic maleic anhydride for reaction with blood type fluids. It is thought that this is done to render the molecule more hydrophilic and as such, the compound no longer functions as an anhydride.
The general approach as set forth in the above patent disclosures involves the thickening or reaction of blood as if it were an aqueous fluid. Inevitably, all of the agents utilized to immobilize the blood samples are cross-linked to some extent and are at least partially insoluble in an aqueous environment due to this cross-linkage. All of the immobilizing agents with the possible exception noted, form class 2 gels due to the three-dimensional cross-linking discussed previously. None of the systems for thickening, however, contemplate the reaction with blood components itself to immobilize the solution. Such a system would allow for much more complete utilization of the material because it would complex with the entire solution of blood or menses rather than be part of the selective immobilization present with the class 2 gels which are part of the conventional approach.